(1) Field of the Invention
The present invention is directed generally to an electrical capacitor, and more particularly to a power capacitor in an oil filled housing.
(2) Description of the Related Art
An oil filled capacitor is disclosed, for example, in German Application No. D-28 25 377 C2 which corresponds to U.S. Pat. No. 4,296,453.
During manufacture of the known capacitors, one or more windings are stacked on top of one another as needed, possibly separated by intermediate caps. The windings are wired and inserted into metal tubes as a winding packet closed with a lower insulating cap and with an upper insulating cap. To prevent partial electrical discharges, the capacitors are then impregnated with oil where the quantity of oil after impregnation is set so that a buffer gas volume is present in the housing but so that the uppermost winding together with the upper insulating cap is immersed below the level of the oil.
It is necessary to provide the buffer gas volume in cylindrical housings to absorb the increase in pressure due to expansion of the material at elevated temperatures since the wall of the housing does not yield. Yielding of the floor and cover are not adequate to compensate for the expansion since each of these only comprises an extremely small portion of the surface.
Furthermore, the gas bubble must not be situated only above the winding where the insulation cap for the winding is placed.
In terms of order of magnitude, the dielectric strength, or breakdown electric field, of a solid insulator lies at about 200 volts per micrometer, while that of a liquid insulator lies at about 20 volts per micrometer and for gases the dielectric strength only amounts to about 2 volts per micrometer. Accordingly, the electrical insulation between the housing and the winding is weakened by the lower dielectric strength of the gas when the gas bubble displaces the impregnation oil at a critical location.
Since, for example, capacitors are stored horizontally during transport, there is a risk that the gas bubble will migrate under the upper insulation cap. When the capacitors are again placed upright, such as for mounting this gas bubble will remain under the insulation cap and, in the worst case, will fill out the entire volume under the cap. It is not possible for the gas bubble to leave this location so that the upper end of the capacitor winding, which generally has a metallic contact or schoopage, layer is no longer situated under the oil but is exposed by the gas bubble. Partial electrical discharges can arise from the outside edge of the winding under the cap to the housing given alternating voltages of greater than about 1 kilovolt.
These partial discharges can be identified, for example, by measuring a capacitance C.sub.BG between the shorted metallic coatings on the winding and the housing. The current is then calculated at a test voltage U.sub.P. When the current is greater than corresponds to the capacitance according to the equation I.sub.P =U.sub.P .multidot..omega..multidot.C.sub.BG (.omega. is the frequency of the test voltage), this indicates the presence of partial discharges.
To eliminate the described difficulties, there have been previous attempts to grind off or file off the outside edges of the end contact layers from the blank winding layer at the end, or to increase the distance from the housing by applying a wrapping of plastic or paper having a thickness of between 1 to 3 micrometers. Despite all these measures, one must still accept an up to threefold increase of the insulation current due to partial discharges as the result of economical reasons.